During the summer, the Terrasse St-Ambroise is my favourite place in the city. Beyond the normal diversity of people you would find on any montreal patio, the addition of kids and dogs enjoying the terasse gives the place a special community vibe that is as rare as it is wonderful.
The limited menu of BBQ available is great both in terms of taste and price, and is readily available when you find yourself a bit peckish.
Follow the terrasse on facebook as they have special events and movie nights.
Watching Point Break on a warm summer night, projected onto a delivery truck with tragically wonderful french subtitles has been one of my favourite experiences in the city and could only be had in Montreal (the disparity between the rendering of colorful metaphors in the english dialogue and the French subtitles adds an extra layer of humour).